1. Field of the Invention
The present invention relates to a process for the preparation of aryl cyanates by reaction of polyhydric phenols with cyanogen chloride or bromide in the presence of tertiary amines.
The cyanic esters of polyhydric phenols, referred to in short as aryl cyanates here and below, are starting materials for the preparation of high-temperature-resistant thermosets with a triazine structure (see, for example, DE-A 17 20 663). Here and below, polyhydric phenols are taken to mean, in particular, those compounds in which two or more benzene rings, each of which is substituted by a hydroxyl group, are bonded together via direct bonds, lower di- or polyvalent acyclic hydrocarbon radicals, di- or polyvalent mono- or polycyclic aromatic or cycloaliphatic radicals, heteroatoms, or complex groups composed of the aforementioned radicals. Examples include the various compounds known as bis- and triphenols, such as bisphenol A, and the oligomeric condensation products of formaldehyde and phenols or cresols known under the name novolaks.
2. Background Art
The aryl cyanates mentioned are usually prepared by reacting the corresponding polyhydric phenol or its adduct or salt with a tertiary amine, for example triethylamine, in a nonaqueous solvent such as, for example, acetone, with a cyanogen halide, in particular cyanogen chloride or bromide, at a very low temperature (see, for example, DE-A 195 19 102). The hydrogen halide which forms is bound by the tertiary amine which is present in the phenol-amine adduct or, when a free phenol is used, has been added as such. This produces molar amounts of a tertiary ammonium halide, all of which must be removed, particularly when the prepared aryl cyanate cannot be purified by crystallization or distillation. It is usually removed by extraction with water, optionally with prior filtration or centrifugation of precipitated ammonium halide. However, the only solvents which are highly suitable for this purpose are those which are immiscible with water. The reason for this is that if water-miscible solvents such as, for example, acetone are used, aqueous solvent mixtures are produced which can only be worked up again with great difficulty and must therefore in most cases be disposed of as waste material. However, even when water-immiscible solvents are used, problems frequently arise during phase separation as a result of traces of solvent which remain in the water of extraction and as a result of traces of water in the organic phase, which can make additional drying necessary.